Layout Debugger

Overview

The layout debugger can dump a variety of things including:

• Matched CSS Rules

• Style Sheets

• Content (DOM) tree

• Text Runs

• Frames

• Retained display list

Usage

• Use a debug build or add ac_add_options --enable-layout-debugger to your mozconfig.

• Run with ./mach run --layoutdebug [filename]. Dump information via Menu -> Dump.

Command-line options

• Example ./mach run --layoutdebug [filename] --dump-frames

Option	Description
--layoutdebug [<url>]	Start with the Layout Debugger.
--autoclose [<seconds>]	Automatically close the Layout Debugger once the page has loaded, after delaying the specified number of seconds (defaults to 0).
--capture-profile [<filename>]	Capture a profile of the Layout Debugger using the Gecko Profiler, and save it to the specified file (defaults to profile.json).
--paged	Layout the page in paginated mode.
--anonymous-subtree-dumping	Toggle option to include anonymous subtrees in content dumps.
--deterministic-frame-dumping	Toggle option to only include deterministic information in frame dumps, for ease of diffing.
--dump-content	Dump the content tree after page load.
--dump-frames	Dump the frame tree (in app units) after page load.
--dump-frames-css-pixels	Dump the frame tree (in CSS pixels) after page load.
--dump-retained-display-list	Dump the retained display list after page load.